Maser superconductive magnetic



March 5, 1963 P. F. CHESTER 3,080,527

MASER SUPERCONDUCTIVE MAGNETIC FIELD PRODUCING MEANS Filed Jan. 50, 1959 Fig. l 2 s r /L ]'1 WITNESSES INVENTOR KW Peter F. Chester BY QMQ 1% 7 M ATTORNEY physical dimensions of the maser apparatus. lower radio frequencies this external magnetic field is 3,030,527 MASER SUPERCONDUCTIVE MAGNETIC FIELD PRODUCING MEANS Peter F. Chester, Pittsburgh, Pa, assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Jan. 30, 195$, Ser. No. 790,286

5 Claims. (Cl. 330- 4) This invention relates to improvements in maser apparatus, and more particularly to improvements in maser apparatus of the type in which the active material, for

- mined by the operating radio frequency of the maser, the

characteristics of the paramagnetic material, and the \At the normally supplied by a solenoid and a regulated current supply. A further requirement of masers of this type is a relatively high Q resonator tuned to the radio frequency of the signal to be amplified.

It has been a common practice in prior art masers to employ liquid helium for producing the low temperatures approaching absolute zero which are desirable, and in some cases necessary, for the operation of solid state masers. The use of very low temperatures is desirable because it allows amplification with very low background noise and an accordingly increased signal-to-noise ratio.

Prior art masers, however, have the disadvantage that they are heavy and bulky, and the apparatus of the instant invention overcomes these and other disadvantages of the prior art.

In summer the apparatus of the instant invention employs a resonant chamber which may be a circular waveguide section preferably some integral multiple of one-half wavelength long of the radio or microwave frequency to be amplified, having the chamber walls thereof composed of a material adapted at a predetermined critical temperature to assume a superconductive state, and having radio frequency current path means disposed therein also composed of the superconductive material. An external magnetic field is applied by a solenoid or other magnetic field producing means to active material located within the resonant chamber, and the temperature of the chamber is thereafter lowered through the transition temperature of the material to a suitable working temperature. At this time the magnetic field is locked in the resonant chamber, and the external magnetic field producing means may he de-energized and removed.

Accordingly, a primary object of the invention is to provide new and improved maser apparatus having a resonant chamber composed of superconductive material for locking in a magnetic field externally applied to paramagnetic material located within the resonant chamber.

Another object is to provide a new and improved maser device in which external means for creating a steady state magnetic field may be dispensed with for long periods of time.

Other objects and advantages will become apparent atent after a study of the following specification when read in connection with the accompanying drawings, in which:

FIGURE 1 is a view of the apparatus of the instant invention according to the preferred embodiment thereof; and

FIG. 2 is a view of a portion of the apparatus of FIG- URE 1 as modified in a second embodiment of the invention.

Referring now to the drawings, in which like reference numerals are used throughout to designate like parts for a more detailed understanding of the invention, and in particular to FIGURE 1 thereof, there is shown at 10 a substantially cylindrical (or other cross section according to mode of resonance) resonant cavity having the walls thereof composed of a material which may assume a superconductive state. Among suitable materials for constructing the cavity forming means 10 are lead, niobium and vanadium. The phenomenon of superconductivity has been extensively treated in scientific literature and need not be described in detail here. It is sufiicient to state that all of the three materials mentioned, when cooled to a temperature approaching 4.2 absolute, at

some temperature critical to the particular material and the magnetic field on it, assume a superconductive state in which their normal resistance vanishes. Axially mounted within the chamber 9 formed by cylindrical sec tion Mi is a central conductor 11 also composed of a superconductive material, the upper end of the conductor 11 being supported by four very thin vanes 12, 13, 14 and 15, FIG. 2, and the lower end of the conductor 11 being supported by four very thin vanes 16, 17, 13 and 19, FIG. 2. All of the vanm are also composed of a superconductive material and preferably the Wall of the resonant chamber, the vanes, and the central conductor 11 are all composed of the same material. The planes of the sides of the vanes are parallel to the longitudinal axis of the section 10. Mounted within the resonant chamber 9 is an element of active paramagnetic material 20 having a bore 21 therein through which the center conductor 11 passes.

The active material 26 may be any suitable paramagnetic material suitable for having induced therein a state which will permit microwave amplification by stimulated emission of radiation. As is Well known, in some materials including paramagnetic materials the electrons of an atom or molecule have differing energy states, some of the electrons occupying low energy spin states and others of the electrons occupying higher energy spin states. The operation of the apparatus of FIG. 2 will be first described with reference to a paramagnetic material having two electron energy levels. In accordance with the temperature of the active material 26 there is a normal distribution of the electron population between the two energy states. in preparing the material for microwave amplification by stimulated emission of radiation, a process generally termed state preparation, there is induced in the material a temporary excess of electrons having high energy spin states, and when electromagnetic wave energy of suitable frequency impinges upon the active material, the electrons with the high energy spin states give up their excess energy to the radio frequency electromagnetic wave thereby amplifying the radio frequency wave, the electrons thereafter returning to their lower energy states, State preparations may be provided by applying pulses of desired duration and amplitude of radio frequency energy of predetermined microwave frequency into the cavity, the

predetermined frequency being chosen in accordance with the strength of an external magnetic field applied to the paramagnetic material. After state preparation has been achieved, there is applied to the cavity electromagnetic wave energy to be amplified, the amplification process taking place as described hereinbefore. As will be readily understood by those skilled in the art, the interval between the state preparation and the application of microwave radio frequency energy to be amplified must be smaller than the normal relaxation period of the active material.

The apparatus of FIG. 1 is also suitable for state preparation by supplying the cavity with pulses of microwave energy which sweep over a predetermined frequency range including the resonant frequency of the paramagnetic material.

Connected to the chamber forming means or wave guide sectionlfi' is a coaxial cable generally designated 22 for conducting radio frequency energy to and from thechamher 9, the coaxial cable generally designated 22 having an outer conductor 23 which is electrically connected to the wall of chamber forming means 10, and havingif desired a dielectric portion 24, and an inner conductor 25.

The inner conductor25 terminates in a coupling loop 2-6 which extends between vanes 14 and 15, FIG. 2, and connects to the wall of the chamber forming means at connection 27. It will be readily understood by those skilled in the art that the coaxial cable 22 may be connected by way of various coupling and decoupling devices of any convenient design to a source of state-inducing microwave radio frequency energy, to a source of microwave radio frequency energy to be amplified, and to a utilization device for utilizing the amplified radio frequency energy.

Disposed around the outside of the cavity forming means or waveguide section 10 is a cryostat generally designated 28 which has outer and inner walls 29 and 30 with an evacuated space 31 therebetween. The cryostat 28 has a cover portion 32 for sealing the chamber formed within the cryostat, and the cryostat 28 is shown partially filled with a cooling medium 33, for example, liquid helium, the helium 33 being injected into the cryostat through the opening 34 which may have cover means 35 having a small vent 36 therein. It will be noted that the liquid helium 33 also fills the waveguide section 10 which has open ends, but the operation of the apparatus is not impaired thereby. It is essential that the level of the cooling liquid 33 inside the cryostat 28 be maintained at a sufiicient height so that the waveguide section It) is completely filled, otherwise the apparatus would become detuned, as will be readily understood by those skilled in the art. The waveguide section 10 preferably has a longitudinal dimension substantially equal to one-half wavelength of the radio frequency energy to be amplified while filled with liquid helium or any other coolant which it is desired to use at 33, or some integral multiple of one-half wavelength. Preferably the ratio of length to diameter of sectionltl is large.

Disposed around the outside of the cryostat 28, FIG. 1, is a large electromagnet 37, shown in the form of a solenoid in FIG. 1 for reasons of simplicity, but it should be understood that the electromagnet 37 may comprise a large number of layers in accordance with the necessary strength of the magnetic field and the required number of ampere turns needed for setting up the magnetic field.

Electromagnet 37 is connected by lead 3% to a rheostat 39 for adjusting the current in the electromagnet 37, rheostat 39 being connected by lead 40, switch 41 and lead 42 to one terminal of a battery or other source of direct-current potential 43. The other terminal of the battery 43 is connected by way of lead 44 to the other terminal of the electromagnet 37.

In the operation of the above-described apparatus, the equipment may be initially put in condition for operation with the cryostat 28 substantially empty and the electromagnet 37 energized by a current of a selected value to set up a magnetic field of predetermined strength and direction substantially parallel to the center conductor 11 and passing through the active material 20, which may be, for example, ruby, or other suitable paramagnetic mate rial described in the scientific literature. While the electromagnet 37 is maintained energized, liquid helium is admitted into the cryostat to a level at least covering the resonant waveguide section 16', with the result that the superconductive material of which elements 10, 11, 12, 13, 14, 15, 16, 17, 18 and 19 are composed is reduced in temperature to a value approaching absolute zero, and when the transition temperature of the material is passed, a transition takes place and the material becomes superconductive. Super currents are set upin the cylindrical shell 10 which maintain .the lock'ed-in magnetic field through ma terial 20 at the same value as the applied field from solenoid 37. Said superconducting magnet means with the locked in-magnetic'field is sometimes referred to as a superconducting magnet operating in 'the persistent mode.

The solenoid 37 -may be thereafter deenergized by opening switch 41, and if desired electromagnet 37 may he slipped off the lower end of the cryostat 28 to reduce the weight of the mas'er apparatus. 7

The fact that theresonant cavity is made of a material which may become superconductive results in the cavity having a veryhigh Q after superconductivity has taken place.

Particular reference is made now to FIG. 2, which is similar to the same portion of FIG. 1, having in addition a small solenoid 45 wound around the paramagnetic active material 20, the solenoid 45 being connected by leads 46 and 47 to a suitable source of direct-current potential, not shown, which may have any convenient means, not shown, for adjusting the current in the solenoid 45 to thereby vary the strength of the additional magnetic field applied toactive material 20. As will be readily understood by those skilled in the art, the frequency at which the electrons of paramagnetic material will give up energy to radio frequency energy to amplify the same may be varied within limits by varying the strength of an external magnetic field applied to the paramagnetic material, and accordingly, the small solenoid 45 provides means for tuning the resonant frequency of the material 20 within predetermined small limits. Preferably the solenoid 45 should not be so large as to interfere with the normal electromagnetic wave energy field within the resonant cavity forming means 10. Preferably the wire of which solenoid 45 is wound is composed of superconductive material, and preferably radio frequency chokes, not shown, are inserted in leads 46 and 47 to prevent loading the cavity, and prevent losses.

As a general rule it may be stated that the higher the critical temperature of the material employed for the resonant chamber forming means, supporting vanes, and center conductor 11, the larger will be the magnetic field which can be frozen in at the time the transition to a superconductive state takes place.

By suitable choice of materials and construction of parts, apparatus may be easily obtained in which thesteady state magnetic field remains frozen in for 24 hours, because the apparatus need be recharged with liquid helium only once every 24 hours, or possibly at even longer intervals.

vAs previously stated, a paramagnetic material-having three or four electron spin energy levels may be employed at 20, in which case state preparation of the material 20 may be obtained by pumping in microwave energy of one frequency, and supplying microwave energy of a second frequency to be amplified. State preparation by pump-mg of a paramagnetic material having three or four electron energy levels is described in the literature, including an article entitled Proposal for a New Type Solid State Maser by Bloembergen, Physical Review, October 15, 1956, and reference may be had to an applicaaccuser P. F. Chester, Serial No. 679,029, filed August 19, 1957,

and assigned to the assignee of the instant application. Where two microwave frequencies are employed, the resonant chamber means should be constructed and arranged to be resonant at both frequencies.

The apparatus of the instant invention has the further advantage that the locked in or frozen in" magnetic field maintained by supercurre-nts in the shell will not be affected by external perturbing fields.

The orders of magnitude of fields which maybe locked in are at least 2,000 gauss for a niobium resonator, and at least 500 gauss fora lead resonator.

, In the apparatus of FIG. 1, it will be understood that in effect the tour vanes at each end of section 10 close the cavity to microwave energy, and that the mode of propagation of the radio frequency in section 10 is any mode which can occur in a coaxial type wave energy condoctor, the member 11 being the center conductor for radio frequency energy. The lines of force of the steady state externally applied magnetic field pass between the thin vanes at each end and are not substantially affected thereby.

If desired, the supporting vanes may be omit-ted from one end of resonant cavity forming means 10, in which case the cavity may be any multiple of one-quarter wavelength long, with the vanes in the end at which the coaxial cable is connected.

Other arrangements of coil 45 are possible.

It will be readily understood that if a very high cavity Q is not desired, center conductor 11 may be an ordinary wire rather than composed of a material capable of a transition to a superconductive state.

Member 10 may have other shapes than round, for example, square or hexagonal.

Whereas the invention has been shown and described with respect to two embodiments thereof which give sat isfactory results, it should be understood that changes may be made and equivalents substituted without departing from the spirit and scope of the invention.

I claim as my invention:

1. Microwave apparatus comprising a sample of active paramagnetic material capable of amplification by stimu lated emission of radiation, superconducting magnet means operating in the persistent mode operably associated with said sample for producing a biasing magnetic field in said sample in a predetermined direction and of at least such intensity as to produce sufficient separation of two energy spin states as to cause said sample to exhibit a negative temperature at a particular frequency, means for cooling said sample and said magnet means to maintain said magnet means in the persistent mode, and means coupled to said sample for impressing electromagnetic wave energy fields on said sample.

2. In apparatus for amplifying radio frequency energy by stimulated emission of radiation, in combination, radio frequency energy containing means, means operatively connected to said containing means for conducting radio frequency energy to and from the containing means, paramagnetic material positioned within the containing means, and removable magnetic field producing means disposed external to the containing means and adapted to subject the paramagnetic material to a steady magnetic field of predetermined direction and of at least such intensity as to produce sufficient separation of two energy spin states in said paramagnetic material as to cause said paramagnetic material to exhibit a negative temperature at a particular frequency, said containing means being constructed of a material which becomes superconductive when cooled to a temperature below a transition temperature, means for cooling said containing means and said paramagnetic material to a temperture below said transition temperature, said containing means after the transition to a superconductive state having super currents set up therein which produce and maintain the steady -magnetic field on the paramagnetic material after the removable magnetic field producing means has been removed.

3. In apparatus for amplifying radio frequency energy by stimulated emission of radiation, in combination, radio frequency energy containing means, means operatively connected to said containing means for conducting radio frequency energy to and from the containing means, paramagnetic material positioned within the containing means, magnetic field producing coil means disposed external to the containing means, energizing means connected to the coil means, said coil means while energized subjecting the paramagnetic material to a steady magnetic field of predetermined direction and of at least such intensity as to produce sufiicient sepanation of two energy spin states in said paramagnetic material as to cause said paramagnetic material to exhibit a negative temperature at a particular frequency, said containing means being constructed of a material which becomes superconductive when cooled to a temperature below a transition temperature, and means for cooling the containing means and the paramagnetic material positioned therein to a temperature below said transition temperature, said containing means after the transition to a superconductive state having super currents set up therein which produce and maintain the steady magnetic field on the paramagnetic material when the coil means is deenergized.

4. In electromagnetic wave energy apparatus utilizing stimulated emission of radiation, in combination, resonant chamber forming means composed of a material which becomes superconductive at a predetermined low temperature, said chamber forming means being adapted to have located therein an active material adapted to provide stimulated emission of radiation, said chamber forming means being adapted to have electromagnetic wave energy conducted thereto and therefrom, magnetic field producing means for temporarily setting up a steady state magnetic field of predetermined direction and of at least such intensity as to produce suificient separation of two energy spin states in said paramagnetic material as to cause said paramagnetic material to exhibit a negative temperature at a particular frequency, in said chamber forming means and in said active material, and means for cooling the chamber forming means to a temperature at which superconductivity takes place in the chamber forming means while the steady state magnetic field is being set up, the chamber forming means While in a superconductive state locking in the magnetic field and maintaining a magnetic field applied to the active material after the magnetic field producing means has ceased to produce a magnetic field.

5. Microwave apparatus comprising, in combination, a hollow substantially open ended cavity forming member composed of a normally non-magnetic material which becomes superconductive when cooled to a predetermined temperature in a magnetic field below critical field strength, a sample of active paramagnetic material capable of amplification by stimulated emission of radiation disposed within said cavity forming member, means for coupling microwave energy into and out of said cavity, means disposed in predetermined position with respect to the member for setting up inside the member an applied magnetic field of a predetermined field strength of at least such intensity as to produce sufiicient separation of two energy spin states as to cause said paramagnetic material to exhibit a negative temperature at a particular frequency and less than the critical field strength of said material at said predetermined temperature, said applied magnetic field passing through the member, and means disposed in predetermined position with respect to the member for cooling the member to said predetermined temperature whereat the material of the member becomes superconductive and maintaining the member at a temperature not exceeding said predetermined temperature, so that when the magnetic field is discontinued after said niembe'r rn-a terial becomes superconductive the collapsing OTHER REFERENCES of said applied magnetic field will induce currents in the Winks: qP of e IRE March 1957 paws superconductive member material which set up a magnetic 2914316 field substantially equal in magnltude to the applied Kingston: Proceedings of the IRE May 1958') page magnetic field. 5 916' u v i n n a v McWhorter et aI.: Physical Review, January 15, Referen e; Cltdl 1n the file of ilhlS patent 1958Lpages 312 v318 I I UNITED STATES PATENTS The International Dictionary of Physics and Elec- 2,863,998 Marie Dec. 9, 1958 m mimi s, Van Nostrand 1956, page 563. 7

2,914,736 Young NOV. 24, 1959 

1. MICROWAVE APPARATUS COMPRISING A SAMPLE OF ACTIVE PARAMAGNETIC MATERIAL CAPABLE OF AMPLIFICATION BY STIMULATED EMISSION OF RADIATION, SUPERCONDUCTING MAGNET MEANS OPERATING IN THE PERSISTENT MODE OPERABLY ASSOCIATED WITH SAID SAMPLE FOR PRODUCING A BIASING MAGNETIC FIELD IN SAID SAMPLE IN A PREDETERMINED DIRECTION AND OF AT LEAST SUCH INTENSITY AS TO PRODUCE SUFFICIENT SEPARATION OF TWO ENERGY SPIN STATES AS TO CAUSE SAID SAMPLE TO EXHIBIT A NEGATIVE TEMPERATURE AT A PARTICULAR FREQUENCY, MEANS FOR COOLING SAID SAMPLE AND SAID MAGNET MEANS TO MAINTAIN SAID MAGNET MEANS IN THE PERSISTENT MODE, AND MEANS COUPLED TO SAID SAMPLE FOR IMPRESSING ELECTROMAGNETIC WAVE ENERGY FIELDS ON SAID SAMPLE. 